Harvestmen

Harvestmen belong to the Phylum Arthropoda, Subphylum Chelicerata, Class Arachnida, and Order Opiliones. They represent the third most diverse order in Arachnida. The order Opiliones includes five suborders as follows: Cyphophthalmi, Eupnoi, Dyspnoi, Laniatores, and Tetrophthalmi. They are often referred to as “daddy long-legs.” They are found throughout the world (except Antarctica), and there are over 6,650 species. Their common name is derived from the fact that they are common during the harvesting season.

They are traditionally akin with the Order Acari (ticks and mites) or the Novogenuata (the Scorpiones, Pseudoscorpiones, and Solifugae); however, others suggest the Opiliones forms a clade with the scorpions and two smaller orders. As such, they are considered the sister group of scorpions. In addition, the Opiliones are a distinct order that is not closely related to spiders.

Despite their long history, few harvestman fossils are known. This is mainly due to their frail body structure and terrestrial habitat, making them unlikely to be found in sediments. As a consequence, most known fossils have been preserved within amber. The oldest ones have been found in the 400-million-year-old Rhynie cherts of Scotland and 305-million-year-old rocks in France. These fossils look amazingly modern, indicating that their basic body shape developed very early on and, at least in some taxa, have changed very little in millions of years.

For years, harvestmen was a scientifically ignored group. About a third of today’s known taxa was described by Carl Friedrich Roewer (1881‒1963) between the 1910s to the 1950s. More recently, James C. Cokendolpher (retired from Texas Tech University), and William A. Shear (Trinkle Professor Emeritus at Hampden-Sydney College) have provided a great deal of information on Opiliones.

Although superficially similar to and often misidentified as spiders (order Araneae), they can be easily distinguished from long-legged spiders by their fused body regions and single pair of eyes in the middle of the cephalothorax. Also, true spiders have a distinct abdomen that is separated from the cephalothorax by a constriction, and they have three to four pairs of eyes, usually around the margins of the cephalothorax. The simplest discernible difference between harvestmen and spiders is that, in harvestmen, the anatomical connection between the cephalothorax and abdomen is broad, so that the body appears to be a single ovoidal structure. They also have no silk glands and therefore do not build webs.

Harvestmen are famous for having stilt-like legs that are extraordinarily long relative to their body size; however, some species are short-legged. Their average body length usually does not exceed 7 mm (0.28 in), and some species are smaller than 1 mm. The largest known species is Trogulus torosus (Trogulidae) of the Balkans, which grows to a maximal length of 22 mm (0.87 in). The body of harvestmen have two tagmata, the anterior cephalothorax or prosoma, and the posterior ten-segmented abdomen or opisthosoma. Unlike spiders, they are not venomous (i.e., they have no venom glands in their chelicerae) and are quite harmless to humans. However, harvestmen do possess prosomatic scent (odoriferous) glands (ozopores) on their cephalothorax and can deter some predators with these noxious secretions. In some species, the fluid ozopores contain noxious quinones. These secretions have successfully protected the harvestmen against wandering spiders, wolf spiders (Lycosidae), and some ants (Formicidae). The exact chemical makeup of these secretions seems to vary from group to group.

Most Opiliones have a single pair of eyes in the middle of the head, oriented sideways (except for Cyphophthalmi, which are located laterally, near the ozopores). However, some species are eyeless, such as those from termite nests and from caves.

Their feeding apparatus is called a stomotheca, and it differs from most arachnids in that Opiliones lack pumping stomachs and can not only consume liquids but also swallow chunks of solid food. The stomotheca is formed by extensions of the coxae of the pedipalps and the first pair of legs. Many species are omnivorous, eating primarily small arthropods and various types of plant material and fungi. On the other hand, some scavenge dead organisms, bird dung, and other types of feces.

Most harvestmen ambush their prey, although some actively hunt. Because their eyes apparently cannot form images, they use their second pair of legs as antennae to explore their environment. Unlike most other arachnids, harvestmen do not have a sucking stomach or a filtering mechanism. Rather, they ingest small particles of their food, thus making them vulnerable to internal parasites such as gregarines. After eating, harvestmen clean their legs by drawing each leg in turn through their jaws.

Interestingly, cases of harvestmen feeding upon live vertebrates (birds) were reported in Arkansas (White River National Refuge) and North Carolina. Nocturnal feeding on Swainson’s warblers (Limnothlypis swainsonii) nestlings contributed to or resulted in the death of one or more nestlings. Another event involved a group of at least four harvestmen with one to four individuals feeding upon a brown-headed cowbird (Molothrus ater) nestling at one time, and the other involved up to two harvestmen present at the nest, but only one individual fed upon the two Swainson’s warbler nestlings.

Predators of harvestmen include a variety of animals, including other arachnids like spiders and scorpions, amphibians, and some mammals. They also are capable of displaying in a variety of primary and secondary defenses against predation, ranging from morphological traits such as body armor to behavioral responses to their chemical secretions.

In terms of reproduction, most harvestmen reproduce sexually, although parthenogenetic species do occur. They have a gonopore on the ventral cephalothorax. Except for some mites, male harvestmen are unique among arachnids in having a penis for direct copulation (sperm transfer), and all females are oviparous (egg layers). The males of some species offer a secretion (nuptial gift) from their chelicerae to the female before copulation. Sometimes, the male guards the female after copulation, and in many species, the males defend territories. In some species, males also exhibit post-copulatory behavior in which the male specifically seeks out and shakes the female’s sensory leg. This is believed to entice the female into mating a second time.

Shortly after mating or ranging up to several months later, females lay eggs from an ovipositor, and some build nests for this reason. One unique feature of harvestmen is that some species engage in parental care, whereby the male is solely responsible for guarding and cleaning the eggs resulting from multiple partners, often against egg-eating females. Depending on temperature and after being laid, the eggs can hatch from between twenty days up to about six months. Most harvestmen have six instars, but some variously pass through four to eight nymphal instars to reach maturity. Most species live for about a year.

For respiration, harvestmen breathe through tracheae but do not have book lungs like spiders. They also have a pair of spiracles located between the base of the fourth pair of legs and the abdomen, with one opening on each side. In many of the on-the-go species, spiracles are also found upon the tibia of the legs.

Most species are nocturnal and colored in shades of brown, although a number of diurnal species are known, some of which have intense patterns of yellow, green, and black with varied reddish and blackish mottling and reticulation.

Many species of harvestmen clearly put up with members of their own species, with aggregations of many individuals often found near water at protected sites. These aggregations can number 200 individuals and, amazingly, up to 70,000 in other groups. Gregarious behavior is likely a strategy against climatic variability, but also against predators, combining the effect of scent secretions, and reducing the probability of any particular individual being preyed upon.

Many species of Opiliones are relatively common and require no protection, but several are threatened or endangered. The latter includes most cave-adapted (troglobitic, troglophilic) species. In Arkansas, members of the genus Leiobunum (Sclerosomatidae) are sometimes observed in entrances of Arkansas caves, and the troglophilic sabaconid harvestman Sabacon cavicolens is also easily found at the same microhabitat. A single family, Phalangodidae, contains troglobites in Arkansas caves. One is an eyeless species, Crosbyella distincta, described in 1942 from a single cave in Boone County, while an eyed species, C. roeweri, is known from a solitary cave in Benton County. Two harvestmen (Leiobunum sp. and Crosbyella spinturnix) have been reported from Foushee Cave in Independence County. The latter, considered a troglophile, has also been reported from Devil’s Den Cave in Washington County and other caves in the state. The more common harvestman in the state is L. politum.

Harvestmen have also been well studied in caves of central Texas and include the Bee Creek Cave harvestman, Texella reddelli, and the Bone Cave harvestman, T. reyesi, both listed as endangered species in the U.S. and ranked S2 (imperiled). Others are the Cokendolpher Cave harvestman, T. cokendolpheri, ranked S1 (critically imperiled), known only from Robber Baron Cave in Bexar County in central Texas.

Several from the vicinity of springs and other restricted habitats of central California are Calicina minor, Microcina edgewoodensis, M. homi, M. jungi, M. leei, M. lumi, and M. tiburona and all are being considered for listing as endangered species, but as yet have received no protection.

There are several pathogens and parasites of Opiliones. They include bacteria, fungi, protists, trematodes, cestodes, nematodes, and arthropods.

For additional information:
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Chris T. McAllister
Eastern Oklahoma State College

Henry W. Robison
Sherwood, Arkansas